In a field of optical information processing and optical measurement, the present invention relates to a system which uses coherent light beams to perform optical correlation processing on two-dimensional images obtained either from photographing devices such as CCD cameras, or directly from objects, and thereby automatically perform pattern recognition and measurement. The invention also relates to a method of driving a ferroelectric liquid crystal spatial light modulator used in this system.
Among conventional optical pattern recognition systems and correlation processing apparatuses, a joint transform correlator is well known. Patents concerning this method include Japan Patent Laid-Open Nos. 138616/1982, 210316/1982 and 21716/2983. One example of a conventional correlator of this method is shown in FIG. 3. In this method, a reference image that provides a reference for recognition and an input image to be recognized are arranged simultaneously side by side to form an object image 124. A light beam emitted from a laser 121 is expanded by a beam expander 122 and then split into two beams by a beam splitter 123. The beam, having passed through the beam splitter 123, irradiates the object image 124 to transform it into a coherent image. The coherent image is Fourier-transformed by a first Fourier transform lens 125 to display a light intensity distribution of a joint Fourier-transformed image of the reference image and the input image on an optically addressed type light valve 126 placed on the transformation plane. The joint Fourier-transformed image is an interference fringe pattern produced by interference between two (or multiple) Fourier-transformed beams of the input image and the reference image and includes conjugate components of these images.
Next, the beam split by the beam splitter 123 is reflected by mirrors 129, 130 and a polarized beam splitter 105 to irradiate the optically addressed type light valve 126, converting the light intensity distribution of the joint Fourier-transformed image displayed into a coherent image. The coherent image is read as a negative or positive image as it passes through the polarized beam splitter 105 that acts as a light detector, and then the image is received by the CCD camera 128 located on the image transformation plane. In this way, a correlation peak representing a two-dimensional correlation coefficient of the reference image and the input image is obtained from the CCD camera 128.
Such optical pattern recognition systems and correlation processing apparatuses have come to require a spatial light modulator having a high resolution and a high-speed response as the research of these devices progresses. The optically addressed type spatial light modulators that have been in common use include those using an electric-optical liquid crystal such as Bi.sub.12 SiO.sub.20 crystal and a liquid crystal light valve using nematic liquid crystal. These, however, do not fully satisfy the requirements such as resolution and high-speed responsiveness. Under these situations, a new optically addressed type spatial light modulator (FLC-OASLM), which uses a ferroelectric crystal as a light modulating material, have been developed and come to be used.
As shown in FIG. 3, however, since the conventional joint transform optical correlators are required to use two Fourier transform lenses 125, 127 to produce a joint Fourier-transformed image of the reference image and the input image and to produce correlation peaks that correspond to two-dimensional correlation coefficients between the reference image and the input image, the system becomes very large. It may be possible to reduce the size of the system by using an optically addressed type spatial light modulator of a transmission type that uses Si.sub.12 SiO.sub.20 crystal, instead of an optically addressed type liquid crystal light valve, to make one and the same Fourier transform lens work as two different Fourier transform lenses. The optically addressed type spatial light modulator using the Si.sub.12 SiO.sub.20 crystal, however, has a drawback of being unable to directly modulate and operate the laser diode at high speed because of the modulator's low write sensitivity to the oscillation frequency of the laser diode, and because of its high drive voltage of several kV. The optically addressed type spatial light modulator with the Si.sub.12 SiO.sub.20 crystal has a low resolution of 15-30 lp/mm, so that forming a small object image requires either putting the reference image and the input image very close together or using a Fourier transform lens with a very long focal length, making it practically impossible to reduce the size of the system.
Further, since the conventional optically addressed type spatial light modulator takes about 100 msec to record, read and erase a picture, it is difficult to operate at high speed an optical recognition system using the optically addressed type spatial light modulator.
The FLC-OASLM with no light reflection and separation layers has the following problems. Because it has no dielectric mirrors or light shielding layers, the writing light and the reading .light cannot be separated and the reading light has adverse effects on a photoconductive layer. Even when the FLC-OASLM has light reflection and separation layers, if the reading light is too strong or the transmission factors of the dielectric mirror and the light shielding layer are not sufficiently small, the reading light will adversely affect the photoconductive layer. That is, when a strong reading light is radiated when the FLC-OASLM is being applied with writing pulses, the reading light acts on the photoconductive layer in the same way as the writing light to induce the inversion of the ferroelectric crystal molecules, resulting in a failure of an image being recorded to be written correctly or at all. To avoid this, the conventional practice is to use a very weak reading light of an intensity that will not adversely affect the photoconductive layer.
However, when a weak reading light is used, the light intensity of an image being actually read, though it depends on the wavelength of the reading light, is very small because the FLC-OASLM has no ferroelectric mirrors and the reflection factor of its reading light is low. As a result, at the next step where the image thus read out is optically processed and recorded again in another spatial light modulator, the light intensity of the image may become so weak as will make the next step processing difficult or impossible.
A further disadvantage is that the writing characteristic such as sensitivity and resolution of the FLC-OASLM varies greatly depending on the strength of the reading light.